1. Field of the Invention
This invention relates to an improvement in the batch melt polymerization process for preparing copolyamides consisting essentially of randomly recurring units of the formulas ##STR1## where 45 to 80% of the units are 6TA units. Copolymers consisting essentially of 6TA and 6IA units are referred to herein as 6TA/6IA copolymers.
2. Description of the Prior Art
In the batch melt polymerization process for preparing a 6TA/6IA copolymer, an aqueous solution of an appropriate mixture of hexamethylene diammonium terephthalate (6TA salt) and hexamethylene diammonium isophthalate (6IA salt) is charged to a vessel (e.g. autoclave). The vessel is sealed from the atmosphere. The solution is then heated with stirring or agitation under controlled conditions of time, temperature and pressure to remove water from the vessel, effect polymerization of the salts and provide the 6TA/6IA copolymer in the molten state. Normally the copolymer is then extruded from the bottom of the vessel, diced, stored and subsequently remelted and shaped into fibers or molded articles. A minor amount (10 to 35%) of the polymer remains on the inner surfaces (e.g. walls and agitator) of the vessel and is referred to as the heel. The process is then repeated to prepare successive batches of the copolymer. If the salt mixture contains less than about 40% by weight of the 6TA salt, the entire heel in the vessel from the last batch melts and blends with molten copolymer formed in the next batch to provide homogeneous copolymer having substantially the same properties and appearance as the copolymer prepared in the previous batch. However, if the salt mixture contains more than about 40% by weight of the 6TA salt, the entire heel does not melt in the molten copolymer formed in the next batch but rather a portion of the heel disperses in the copolymer of the next batch in the form of opaque particles of noticeable size to provide a blend of heel and copolymer from which useful fibers and articles can not be shaped. Moreover, a sufficient amount of heel particles accumulate in the vessel after several successive batches, if not after the first batch, to cause plugging of extrusion passages leading from the vessel, thereby making it impossible to extrude the blend of heel and copolymer from the vessel. The only way to effectively remove the heel/copolymer blend from commercial type vessels is by dismantling and then cleaning the heel from the dismantled parts of the vessel. Such a procedure is time consuming and costly and, therefore, not practical or feasible for commercial operations.
U.S. Pat. No. 4,238,603 discloses that the above mentioned heel problem associated with using the batch process to prepare successive batches of 6TA/6IA copolymer high in 6TA content can be overcome by modifying the 6TA/6IA copolymer to provide a terpolymer, for example, the terpolymer formed by polymerizing a mixture of 6TA, 6IA and nylon 66 salts where from 2-15% of the mixture is nylon 66 salt. These terpolymers, however, require an additional monomer, the presence of which alters the properties of the 6TA/6IA copolymers, for example, the melting point, crystallinity and tensile strength thereof.